Rhodococcus is a genus of aerobic actinomycetes with characteristics similar to those of Nocardia. Morphologically the rods vary from cocci to long, curved, clubbed forms.Some strains are acid-fast. Rhodococcus has recently been recognized as an opportunistic pathogen causing an aggressive pneumonia in severely immunocompromised patients, particularly those with acquired immunodeficiency syndrome. The organisms are found in the soil. One species, Rhodococcus equi, has an association with horses where it also causes pneumonia in foals. This species is a facultative intracellular pathogen of macrophages with features somewhat similar to those of Legionella and Listeria. Optimal treatment is unknown, although erythromycin, aminoglycosides, and someβ-lactams show in vitro activity.